Eight-meter-wavelength Transient Array (ETA)
Science Primer
The ETA is a radio telescope designed to
observe the short duration radio pulse --- the radio transient --- that
is expected to be produced by a number of high-energy astrophysical
phenomena (exploding primordial black holes, gamma ray bursts,
supernovae, and compact object mergers). The ETA will search for these
transients during continuous observation of nearly all of the northern
hemisphere of the sky. Each of these astrophysical phenomena are
discussed below. Of course, new classes of objects may also be
discovered.
This page is aimed at readers with an "intermediate" level of
understanding of the concepts. "Beginners" may find it helpful to
consult the glossary of terms for unfamiliar concepts and objects. Links
to the glossary are scattered throughout this document. "Advanced"
readers may find this document a useful summary of the science.
Table of Contents
Giant Pulses --- The Crab Pulsar
Pulsars typically emit pulses of
roughly constant magnitude. However, a handful of pulsars are known to
emit an occasional "giant pulse" (GP). The Crab Pulsar (PSR B0531+21) is
the best known of these pulsars, with giant pulses of 10 to 1000 times
the mean pulse intensity. Indeed, the Crab Pulsar was accidentally
discovered, in the late 1960s, through the detection of its giant pulses
at 81 MHz. Some of these giant pulses have been observed to have
subpulses that last no longer than a few nanoseconds, indicating the
emission region is no larger than a few nano-lightseconds in size (a few
feet in size). At radio wavelengths, only the Sun appears brighter than
the Crab pulsar during one of these "nano-giant pulses." We expect the
ETA will be able to detect the Crab's giant pulses, providing a useful
diagnostic for the system. Of course, we may also detect giant pulses
from other pulsars. These extreme emission events currently defy
explanation; any additional observations of giant pulses could be useful
in pinning down possible models.
Exploding Primordial Black Holes
"Primordial" black holes of a
range of sizes may be produced as a by-product of the density
fluctuations in the Big Bang. If
black holes evaporate as suggested by specific combinations of general
relativity and quantum mechanics, then those primordial black holes with
masses below about 10^14 g will evaporate in about 10^10 years. There
may be as many as 10^23 of these black holes in our Galaxy alone. The
evaporation process quickens as the mass of the black hole decreases and
the process terminates in an explosion releasing 10^30 erg or more of energy in much less than 1
second. The resulting relativistic expansion of charged particles could
interact with the ambient interstellar magnetic field to create an
electromagnetic pulse of length approximately 1s that could be detected
in the radio spectrum. Given a reasonable value of about 10^5 for the Lorentz factor of the
relativistically expanding fireball, the resulting radio pulse could be
detectable at wavelengths on the order of 1 m by a single nondirectional
antenna up to a distance of about 10^4 parsecs (10^4 pc). The 12-antenna ETA
may be able to detect such a pulse from an exploding primordial black
hole in our Galaxy. A discovery would be enormously significant as a
test of our current understanding of quantum and gravitational physics.
However, even a nondetection with improved limits established on the
rate of explosions would be useful in further constraining the spectrum
of density fluctuations in the early universe.
Gamma-Ray Bursts
Gamma-ray bursts have been
observed, but these enigmatic objects are yet unexplained. These short
duration events are undoubtedly due to high-energy events. Fading
optical emission and even radio emission has been observed from such
events, but prompt radio emission from these events would be very
useful in pinning down the physics of the bursts, the nature of the
progenitor object, and possibly the medium in which it occurs. If these
phenomena occur at large redshifts,
there is the possibility that the observations could probe the Epoch of Reionization, or the intergalactic medium. A
number of models have been proposed to explain the gamma-ray bursts,
ranging from compact object mergers (see below), to maser-like coherent
emission. These models are not well constrained by current observations.
Supernovae
As for primordial black holes, the violent expansion of charged
particles, from a supernova, into
the ambient magnitude field would produce a pulse that might be
detectable by the ETA. Approximately one supernova event per century is
expected in a galaxy.
Compact Object Mergers
Binary star systems consisting of closely separated compact objects such
as neutron stars and/or black holes will eventually merge as
a result of the emission of gravitational radiation (and due to any
other energy loss). Such mergers will potentially produce a burst of
emission that could be detected in the radio spectrum. Several mergers
involving a pair of neutron stars are expected per year within 5
Megaparsecs. Mergers of a neutron star and black hole are expected to be
more common, but the resulting pulses are expected to be weaker. The
various model parameters assumed in these calculations are very poorly
constrained, and that any number of surprises could result in pulses
which are significantly stronger than projected.
Sensitivity of the ETA
(An "advanced" section)
The 12 dual-polarization dipole antennas have a total effective
collecting area of A = 476 square-meters at the zenith. The system
temperature T is dominated by galactic emission (T is about 9000K, at
the observing frequency of 38 MHz). For the full bandpass of df = 18MHz
sampled at time intervals of dt = 66 microseconds, and adding both
polarizations, the standard radiometer equation yields an rms response
of
rms = T / sqrt (2*12*df*dt) = 9000K / sqrt(2*12*18MHz*66microseconds) = 53K,
where "sqrt" means "take the square root."
We may, therefore, estimate the expected signal-to-noise ratio for
the Crab giant pulses. For one such estimate, we take the flux density
of about S = 100 Jy measured for the Crab at 74 MHz by Rickett and
Seiradakis (1982 Astrophysical Journal, 256, 612). The flux density may
be higher at 38 MHz, or lower, depending on where the turnover in the
Crab spectrum occurs. For a giant pulse of 10 times this flux density,
the antenna temperature Ta would be given by
Ta = 10 S A / k = 350 K,
where k is Boltzmann's constant, and both polarizations are being
added. Thus the expected signal-to-noise ratio would be Ta/rms = 7. Flux
densities up to 1000 times larger than the average flux would yield a
signal-to-noise ratio of up to 700.
In reality, the pulse will be spread out over a time width of the
order of a second by interstellar scattering, which means smoothing and
sampling at dt=1s would be more appropriate, with a reduced rms and
larger signal-to-noise ratio. The ETA design allows for reconfigurable
approaches to sampling, etc.
NSF Acknowledgment and Disclaimer
This material is based upon work supported by the National Science
Foundation under Grant No. AST-0504677. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of
the author(s) and do not necessarily reflect the views of the National
Science Foundation.
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